[toward py3k] rewrite has_key() (part of #2711624)
The `has_key` method is not in Python3K but dict supports __contains__ check for
ages.
import ConfigParser
try:
undo_actions = config.cfgfile_parser.get('MAIN', 'undo-support', False)
except ConfigParser.NoOptionError:
pass # this conf. file was probably already migrated
else:
config.global_set_option('undo-enabled', bool(undo_actions))